This invention relates to an improvement in a process for obtaining free fatty acids from soap stocks, particularly those from alkali refining of vegetable oil. When glyceride oils, e.g., animal fats or vegetable oils, are refined to remove free fatty acids and other impurities by alkali refining, the aqueous alkaline solution which is separated from the bulk of the refined oil contains alkali soaps of fatty acids together with substantial quantities of free fatty acids, glycerides thereof, and various impurities including water-soluble phosphatides such as lecithin. Some of the free fatty acids, glycerides, and impurities ordinarily will be emulsified in the resulting aqueous mixture by the soaps and the phosphatide-type components. This aqueous mixture is commonly known as "soap stock". It can be subjected to further processing to convert the soaps and esters therein into free fatty acids for recovery.
In one such process the soap stock is first saponified with alkali, typically by heating with aqueous caustic soda. The proportion of such alkali used is ordinarily substantially in excess of the stoichiometric amount necessary to convert all free and esterified fatty acids present into alkali metal soaps. The saponified soap stock then is acidulated with mineral acid, typically aqueous sulfuric acid, to liberate fatty acids from their soaps. These free fatty acids are then separated from the resulting aqueous saline phase by decantation or centrifugation. Frequently, such a fatty acid phase is then vacuum distilled to give a fatty acid distillate. In some cases where fatty acid values of consequence remain in the still residue, such residue can be subjected to re-saponification, re-acidulation, re-separation of the resulting fatty acid and aqueous saline phases, and re-distillation of this fatty acid phase to recover additional fatty acid distillate. Substantial quantities of alkali and mineral acid are used in this process and corresponding large quantities of aqueous saline phase are generated for recovery or disposal.
Among the other methods which have been proposed for soap stock processing to reclaim fatty acids therefrom is that of U.S. Pat. No. 3,471,536. Therein soap stock from alkali refining of edible oils and fats is first acidulated, the resulting acid oil phase separated and the glycerides therein hydrolyzed at high temperature in an autoclave with water. The free fatty acids thus liberated are then distilled from this mixture. A main feature of the patent involves treating the hydrolyzed acid oil with certain amines to reduce the amounts of impurities distilled with the fatty acids. The thermal hydrolysis can generate high impurity contents, thereby imparting contamination to the product fatty acid or requiring additional processing for removal, i.e., by the amine treatment of U.S. Pat. No. 3,471,536. There is no saponification step. Advantages of the instant invention over prior proposals include economy, enhanced product quality, and suppression of impurities formation.